1. Field of the Invention
The invention relates to a drill pipe connector.
2. Discussion of the Prior Art
A generic boring rod connector is known from WO96/03605. This connector comprises a journal element that has a tapered external thread section located between an outer shoulder and a journal face, a thread-free section located between the outer shoulder and the external thread section, and a thread-free section located between the external thread section and the journal face. The connector further comprises a sleeve element that can be screwed to the journal element. The sleeve element has a tapered internal thread section located between an inner shoulder and a sleeve face, a thread-free section located between the inner shoulder and the internal thread section, and a thread-free section located between the internal thread section and the sleeve face. The thread-free sections of the journal element and the sleeve element in the region of the outer shoulder, which, when a screw connection exists, are located opposite to each other, are longer than the thread-free sections located in the region of the inner shoulder. The outer shoulder, which, when a screw connection exists, comes into contact with the sleeve face, forms the primary shoulder. The axial forces that occur in the primary shoulder when a screw connection exists are greater than the forces that occur in the inner shoulder. The screw connection is thereby established with at least 50%, preferably 60%, of the yield point of the drill pipe connector.
The outer diameter of the sleeve element is smaller in the region of the thread-free section than in the remaining sleeve element. This cross-sectional reduction allows the thread-free section of the sleeve element to butt together with the axial elongation of the thread-free section of the journal element when the primary shoulder is activated.
The known drill pipe connector has the disadvantage that the stresses that occur during the establishment and use of the screw connection are taken into account exclusively as axial stresses, so that the part cannot be fully used. The screwing moment is too small, so that the danger exists that the abutting surfaces will separate. Such separation reduces the alternate loadability and can lead to a permanent break.
A comparable drill pipe connector is disclosed in DE 32 45 819 B1. This connector has the same features as the drill pipe connector described above. In this connector, too, the outer shoulder forms the primary shoulder and is activated first when a screw connection is established. Moreover, the thread-free sections located in the region of the outer shoulder are longer than the thread-free sections located in the region of the inner shoulder. It is also proposed that the distance from the sleeve face to the inner shoulder be greater than the distance from the journal face to the outer shoulder. As a result, a gap is created when the sleeve face comes to rest on the outer shoulder during the manual establishment of the screw connection.
The aforementioned disadvantage is found in this connector as well, i.e., the overall stress conditions are taken into account only partly, and the part thus cannot be fully used.
This object is attained, starting from the introductory part, in connection with the characterizing features of Patent Claim 1. Advantageous further developments are the subject matter of subclaims.
The core of the invention is that, in contrast to the known embodiments, in the design according to the invention it is the inner shoulder that forms the primary shoulder, and the thread-free sections located in the region of the inner shoulder are at least twice, and preferably three times, as long as the thread-free sections located in the region of the outer shoulder.
This arrangement has the advantage that, due to the long thread-free section of the journal, the tolerances of the drill pipe connection can be better compensated for. Furthermore, when the primary shoulder is located on the inner shoulder, higher forces can be absorbed, because of lower relative stresses. This is true even when the outer abutting surface is larger than the inner abutting surface. As a result, the outer shoulder and the inner shoulder can be optimally used. The prestressing of the inner shoulder results in a defined residual share of forces for the outer shoulder. This residual share can be optimized, taking into account the manufacturing tolerances, and can be distributed, overall, to achieve a higher degree of utilization of the inner shoulder and outer shoulder than has previously been known. All told, this results in higher permissible screwing torque or working torque.
The distance from the journal face to the outer shoulder is greater than the distance from the sleeve face to the inner shoulder. This is true regardless of the tolerances. The long thin lip of the journal is thereby used as a soft spring path for the prestress of the drill pipe connector. Depending on the length tolerances, the inner and outer shoulders are differently stressed, but without exceeding a load of 90% of the yield point. By the interaction of the inner shoulder and the outer shoulder, a torsion moment of approximately 80% of the yield point is attained.
To reduce the total stress, it is further proposed that, while maintaining a rough standard thread for the external and internal threaded sections of the journal element and sleeve element, a steeper angle of between 15xc2x0 and 20xc2x0, instead of 30xc2x0 as per API, be established for the load flank. At an angle of 18xc2x0, for example, the portion of axial force extending in the radial direction is virtually halved. The 45xc2x0 shear surface is also enlarged. Thus, the overall stresses become more favorable, and the part can be more heavily loaded. A further improvement results when some of the moments are absorbed by a deliberate overlap in the thread region near the inner shoulder. This can be attained, preferably, by having different taper inclinations for the respective threaded sections. The taper inclination of the sleeve element is thereby steeper than that of the journal element. The difference in taper inclination is in the range of 0.03 to 0.3 mm/inch. The inner shoulders are relieved by the thread overlap, so that the screwing moment can be further increased.
Another improvement of the design relates to wear on the outer diameter of the sleeve. It is proposed to provide the outer surface of the sleeve, from the face to approximately over the third thread stage, with an internal turning. This ensures that even when wear occurs on the outer diameter of the sleeve, the critical cross-sections, and thus the stress conditions, remain constant. As soon as the region of the internal turning of the sleeve itself shows the first signs of wear, the drill pipe connector must be replaced or, if possible, reworked.